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INTRODUCTION. 



In some localities we have the most positive evidence that the sediments 

 have been deposited in circumscribed areas, as in the case of the Panama con- 

 glomerate and the Salamanca sandstones, and in these examples we have a 

 peculiar and restricted fauna. 



In its far western extension the Chemung group has become a calcareous 

 formation, bearing many of the characteristic forms of Brachiopoda and but few 

 of the Lamellibranchiata so characteristic of the group in the east. 



The Waverly sandstone and its equivalent or representative formations in 

 the west, have not yet been studied with that degree of care and comparison, 

 over wide areas, which are sufficient to warrant generalizations regarding the 

 distribution of the Lamellibranchiate fauna. The species described from 

 widely separated localities are a sufficient evidence that the conditions for the 

 development of such a fauna have existed over a wider extent than in previous 

 periods of geological history. 



In the process of sedimentation and the imbedding of the shells of this class, 

 the valves have, for the most part, been separated ; and the inner surface adher- 

 ing to the rock, it becomes very difficult to ascertain the hinge structure, a 

 character which should be the basis of generic distinction in fossils of this 

 class. 



In far the larger proportion of the specimens of the Lamellibranchiata 

 obtained from the rocks of New York, the shells have been macerated or dis- 

 solved to such a degree that we have barely a film remaining which preserves 

 the external markings, or, in some species, the characters of the interior are 

 visible without having the exterior markings of the shell entirely destroyed. 

 Many of the shells are extremely thin, but some of the stronger forms, in cer- 

 tain conditions of the matrix, have the shell well preserved. In these forms 

 we have often well-marked casts of the interior, and rarely we may obtain 

 the exterior shell in such condition as to show its entire characters. 



In the study of the species of this cl.ass of shells it is not always possible to 

 determine the changes in form and expression, which may be due to physical 

 influences, and in some degree also to the chemical effects operating upon the 

 enclosing matrix. 



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